Synchronization is a commonly used term in communication networks. Synchronization includes frequency synchronization and time synchronization.
The frequency synchronization is also known as clock synchronization, and means that the working clock frequency value of one device maintains a strict fixed specific relation with that of another device being separate from said one device. The strict fixed specific relation is the same or is in a fixed proportion. For example, if device A works at 2048000.000 Hz and device B works at 4096000.000 Hz, device A is synchronized with device B; if device B works at 4096001 Hz, device A is not synchronized with device B. The synchronicity is not completely precise, and allows for a certain range of errors. Generally, the precision is required to be within several Parts Per Billion (ppb).
The pulses transmitted in a digital communication network are Pulse Code Modulation (PCM) discrete pulses which derive from coded information. If the clock frequency of one digital switching device is inconsistent with that of another digital switching device, or, if phase drift and jitters is superimposed after the digital bit streams are impaired by interference in the transmission process, code elements may be lost or repeated in the buffer store of the digital switching system, causing slip impairment in the transmitted bit streams.
The time synchronization is also known as clock phase synchronization, and means that the internal time of one device and that of another device being separate from said one device are the same (in the range of allowable error). Supposing that at specific time, the time recorded by device A is 18:01, and the time recorded by device B is also 18:01, the time recorded by said one device and that recorded by said another device are synchronized. Different demands require different precision of time synchronization. General computer applications need to be accurate to milliseconds, and mobile communication technologies need to be accurate to microseconds.
The time synchronization differs from the frequency synchronization in that: If the time of one device and that of another device are always consistent, this state is called the time synchronization (or phase synchronization); if the time of one device is different from that of another device, but the difference is constant, this state is called the frequency synchronization.
Phase-Locked Loop (PLL) is a commonly used synchronization technology, and has three main functional modules: a frequency division phase discriminator, a low-pass filter, and a voltage-controlled oscillator.
The voltage-controlled oscillator may output clock frequency independently, and change the output frequency according to an input voltage signal. The frequency division phase discriminator is configured to compare the speed of two frequencies. The low-pass filter may convert this speed information into voltage information, and meanwhile filter some jitters out.
Traditional telecom networks have a perfect mechanism for publishing line clock frequencies. A clock source provides an external line interface, and synchronizes the clock frequency of the external interface with its internal frequency (such as atomic clock frequency). A downstream device recovers clock from the line first, and then synchronizes its internal clock with the line clock, and finally synchronizes all line output clocks with the internal clock. In this way, the whole network may be synchronized to be in a same clock frequency.
Ethernet is an asynchronous network, and does not require the same working frequency between separate devices. Definitely, to ensure interworking between devices, the clock frequency difference between devices is limited to 100 ppm, which is 2000 times greater than the clock frequency difference in the traditional telecom network (namely 50 ppb).
IP-based network is an irresistible trend for the future network, so it is necessary to provide synchronization features in IP networks. At present, the IP network should provide performance of 50 ppb clock frequency synchronization and 3-microsecond time synchronization.
BITS is short for Building integrated time system. Building Integrated Time System (BITS) networks can provide clock synchronization function for communication equipment in the equipment room of buildings. A BITS network includes a local clock source and a cabling network in the building. However, a BITS device, in fact, merely refers to a clock source device, which is generally a rubidium atomic clock with GPS calibration in communication applications.
The local clock source inside the BITS device is output to devices requiring clocks through a dedicated interface. A mainstream clock interface is a frequency interface capable of outputting a clock frequency of 2 MHz/2M bits, and is generally known as a BITS interface.
Another interface is One Pulse per Second (1PPS) interface, which is a time interface, namely, outputs a time pulse signal per second.
The BITS interface and the 1PPS interface are key interfaces indispensible to clock devices.
Synchronous Ethernet is a technology to recover clocks by adopting Ethernet link code streams. As mentioned above, Ethernet is an asynchronous system and can work normally without requiring high-precision clocks, so general Ethernet devices do not provide high-precision clocks. But that does not mean the Ethernet is incapable of providing high-precision clocks. In fact, on the physical layer, the Ethernet transmits signals in the form of serial code streams like the traditional optical network device, and a receiver must have clock recovery function. If the receiver does not have the clock recovery function, communication is impossible. In other words, the Ethernet itself is capable of transmitting clocks, but the capability is not put into use.
In a synchronous Ethernet, the transmitter imports a high-precision clock into the physical-layer chip of the Ethernet, and the physical-layer chip uses the high-precision clock to send data. The physical-layer chip in the receiver may extract the clock out of the line code stream, and during this process the clock precision is not impaired. That is the basic principles of the synchronous Ethernet.
Same as traditional telecom optical network devices, the synchronous network technology needs support of the whole network. As long as one device on the clock link does not support the synchronous Ethernet, the downstream devices are all incapable of obtaining the clock frequency. However, almost none of the existing IP networks support the synchronous Ethernet. Therefore, the deployment of a synchronous Ethernet requires replacement of devices throughout the whole network, which is not practicable. Moreover, the synchronous Ethernet supports the frequency synchronization only, but does not support the time synchronization.
Currently, the main solutions to time synchronization on an Internet Protocol (IP) network are: Network Time Protocol (NTP), and a precise clock synchronization protocol of a network measurement and control system (IEEE1588). The NTP is mature and widely supported, but provides precision as low as milliseconds, which is far from enough to meet requirements.
The basic synchronization procedures of the IEEE1588 include: A slave clock sends a time synchronization packet that carries a timestamp to a master clock; the master clock receives the packet and returns a time synchronization response packet, and records the local time at the moment of receiving the packet and the local time at the moment of sending the response packet into the time synchronization response packet; the slave clock calculates the bidirectional delays between the slave clock and the peer clock according to four timestamps; supposing that the bidirectional delays are equal, the slave clock can calculate the difference between the local time and the remote time, and then adjust the local time accordingly until the local time is synchronized with the peer time.
The IEEE 1588 theoretically depends on a prerequisite, that is, the bidirectional delays for a packet are equal. Delay in an IP network may be divided into two types: line delay and device forwarding delay. The line delay is fixed, and the bidirectional delays for the packet are equal after the factor that the fiber path for sending the packet is not equal to the fiber path for receiving the packet is eliminated. The forwarding delay is caused by the devices which store and forward packets (such as routers and switches). The forwarding delay is uncertain, and changes between microseconds and milliseconds. When the IP network is congested, the forwarding delay is even longer.
In order to improve synchronization precision, many devices adopt a processing method of internal shunting. For IEEE1588 packets, the uplink interface extracts time information, and sends the time information to the downstream interface through an out-band channel (bypassing the switching chip), and the packet is sent out of the downlink interface. In this way, the uncertainty of the delay caused by the switching chip is eliminated, and the final precision can meet requirements.
The IEEE1588 needs to be deployed throughout the network. If any device on the clock link does not support the IEEE1588, the downstream devices are incapable of obtaining enough precise synchronization. The upgrade of the existing IP network requires replacement of devices throughout the whole network, which is hard to realize.